Build Modifier Start Delay Driven by Completion of Another Layer's Build Modifier?
You can use the length of the first layer's build modifier to drive the start delay of the second layer's build modifier.
Setup your two modifiers and their layer accordingly.

Right click into the &quot;Frames&quot; field of the first one, choose &quot;Copy as New Driver&quot;, right click into the &quot;Start Delay&quot; field of the second one, choose &quot;Paste Driver&quot;.
When you scrub along the time line, the second layer should start building right when the first one ends. If you change the length of the first build, the second one will get offset by the same amount.

If you want to add another offset to the second layer's build start, right click into the &quot;Start Delay&quot; field, choose &quot;Edit Driver&quot;, change the driver evaluation to &quot;Expression&quot; instead of &quot;Average&quot; and write length + 10 in the field for instance if you want a delay of 10 frames.